Test to Determine Sterile Integrity of Wrapped Medical Products at a Probability of Recontamination of 1: 1,000,000

Abstract

We developed a microbiological test to detect the penetration of airborne microorganisms through the packaging of medical products after sterilization, to meet the requirements of European standard EN 556. We applied this test method to transparent pouches. The microbial-barrier properties of the transparent pouches were determined using the microbial challenge test, in which the package was placed inside an exposure chamber and exposed to a defined aerosol of Saccharomyces cerevisiae. The atmospheric pressure in the chamber was periodically reduced by 0-75 millibars, to simulate weather-dependent pressure changes. Thermoresistant petri dishes filled with nutrient agar were integrated into the transparent pouches before sterilization. The packages were incubated after exposure. They were then opened and examined for colony growth. The number of recontaminated packages per test group (n=50) depended on the microbial bioload (defined as the number of colony-forming units per plate) to which the packages were exposed and on the size and number of decreases in atmospheric pressure. Results of multiple regression analysis showed a significant increase in the number of recontaminated packages in correlation with the product of the values for microbial bioload and the size and number of decreases in atmospheric pressure. When we analyzed the probability of recontamination of wrapped medical devices after 2 reductions in atmospheric pressure (30 millibars each) and with a surface microbial load of 10 colony-forming units per 64 cm(2), we estimated that the frequency of recontamination was 1 : 100,000. Multiple regression analysis showed that the proposed microbial challenge test is suitable to determine the probability of package recontamination at the 1 : 1,000,000 level.